1. Field of the Invention
The present invention relates to a semiconductor capacitor structure, and more particularly, to a metal-oxide-metal (MOM) type capacitor structure composed of a first capacitor and a second capacitor, which has a plurality of symmetrical branch sections forming an interdigitated structure along a plurality of ring contours. The semiconductor capacitor structure has an optimal geometrical symmetry, and thus a better capacitance matching effect and a higher unit capacitance. In addition, a capacitance value ratio between the first capacitor and the second capacitor in the MOM capacitor structure can be adjusted according to varied requirements.
2. Description of the Prior Art
In semiconductor manufacturing processes, metal capacitors constituted by metal-insulator-metal (MIM) capacitor structures are widely applied in Ultra Large Scale Integration (ULSI) designs. Due to their lower resistance, less significant parasitic effect, and absence of induced voltage shift in the depletion region, metal capacitors with MIM capacitor structure are usually adopted as the main choice of semiconductor capacitor designs.
However, since the manufacturing cost for the MIM capacitor structure is very expensive mainly due to the additional mask(s) required in its manufacturing process, and as the cost becomes more significant along with progress of advanced semiconductor manufacturing process technologies, an interdigitated metal capacitor which only requires the metal-oxide-metal (MOM) capacitor structure in the standard CMOS manufacturing process has been developed in accordance with a requirement for a more economical semiconductor manufacturing process technology. Applications of interdigitated metal capacitors have already been disclosed and discussed in various literature, such as U.S. Pat. No. 6,784,050, U.S. Pat. No. 6,885,543, U.S. Pat. No. 6,974,744, U.S. Pat. No. 6,819,542, and Taiwan Patent No. 222,089 (the Taiwan counterpart patent of U.S. Pat. No. 6,819,542), whose contents are incorporated herein by reference.
In addition, because the layout mismatch of a semiconductor capacitor structure is inversely proportional to the square root of a capacitance value of the semiconductor capacitor structure, conventionally a common centroid type layout topology is utilized to form the layout of the semiconductor capacitor structure, for improving the matching performance (i.e., geometrical symmetry) of the layout of the semiconductor capacitor structure, so as to increase the capacitance value of the semiconductor capacitor structure. One can find such a layout topology as shown in FIG. 1, wherein the semiconductor capacitor structure includes a first capacitor C1 and a second capacitor C2.
A multi-common centroid type layout topology is further utilized in the conventional art to form the layout of the semiconductor capacitor structure, for improving such a common centroid type layout topology, so as to reduce the negative effects caused by semiconductor process variation to geometrical symmetry of the layout of the semiconductor capacitor structure. One can find such an improved layout topology as shown in FIG. 2, wherein the semiconductor capacitor structure also includes a first capacitor C1 and a second capacitor C2. Such a multi-common centroid type layout topology, however, increases layout complexity.
However, no matter which of the common centroid type layout topology shown in FIG. 1 and the multi-common centroid type layout topology shown in FIG. 2 is adopted, the capacitance value ratio of the first capacitor C1 and the second capacitor C2 in the semiconductor capacitor structure can only be set at 1:1. However, since there are situations where an application may require a semiconductor capacitor structure with the first capacitor C1 and the second capacitor C2 having capacitance value ratio other than 1:1, while the semiconductor capacitor structure is also required to have the optimal geometrical symmetry and maximum unit capacitance, the aforementioned conventional arts cannot fully address such a contemporary need.